Interpretive Summary: Ways by which plants move in the nursery industry could help spread certain plant diseases these plants may harbor. Plants are brought in from other states for garden centers and brokers, and plants are also shipped to other states. We discuss how the large-scale movement of plants in the nursery industry may contribute to the spread of plant diseases, notably sudden oak death caused by the fungus Phytophthora ramorum. It is currently unknown, but the cause of much conjecture, whether transport of commercially produced plant material will play a role in the dissemination of this pathogen from the region where it now resides (California coast) or from Europe to new areas. We discuss some potential pathways in commercial horticulture through which P. ramorum may potentially move.

Technical Abstract:
Phytophthora ramorum, the causal agent of sudden oak death disease, can infect many species of plants. Under infected oak trees in California, the pathogen has been found on nursery stock such as Rhododendron sp., often causing visual symptoms (leaf spots) which are easily overlooked or mimic those of several other pathogens. Reports of new hosts of P. ramorum have become common. Large scale testing of nursery crops, shipments, or suspect plants is a major task which is complicated by the application of fungicides in some nurseries that may mask symptom expression. There are several pathways in commercial horticulture though which P. ramorum may potentially move. These include commercial propagation of woody plants from seed, tissue culture, or cuttings, through tissue culture propagation methods, and through movement of ornamental plants during holiday periods. However, the relevance of nursery crop transport in the dissemination of P. ramorum is as yet, unknown.